Java bytecode is Turing-complete, so there's nothing explicitly preventing you from implementing many of the advanced features of C# atop the JVM. As the OP sta

Java bytecode is Turing-complete, so there's nothing explicitly preventing you from implementing many of the advanced features of C# atop the JVM. As the OP stated, alternate languages running on the JVM (such as Scala) already do this. The problem is simply that most Java shops use only the officially-blessed Java language. Because Sun has always forced "certified" implementations of the language and platform to conform to their standard, there has been little room for innovation in the source syntax and bytecode format since the release of the first JDK in 1996. In the meantime, programming languages (and the programmers who use them) have moved forward, and Java has failed to keep up.